An inkjet print head with piezo-electric actuators is well known in the art. Such a known print head comprises a number of pressure chambers. Each pressure chamber is in fluid communication with a respective nozzle orifice and each pressure chamber is provided with a flexible wall. The flexible wall is operatively coupled to a piezo-electric actuator. Upon actuation, the piezo-electric actuator deforms, thereby deforming the flexible wall resulting in a volume change of the pressure chamber. In operation, the pressure chamber is filled with a liquid such as ink and due to the induced volume change, the pressure in the liquid changes resulting in a pressure wave in the liquid. The resulting pressure wave is designed to result in expelling a droplet of the liquid through the respective nozzle orifice.
In a particular piezo-electric actuated inkjet print head, the piezo-electric actuator is a bimorph actuator. Such a bimorph actuator is formed by layered structure comprising a membrane, a bottom electrode, a top electrode and a piezo-electric material layer, wherein the piezo-electric material arranged between the bottom and the top electrode. When a voltage is applied over the bottom electrode and the top electrode, the piezo-electric material deforms. In particular, the piezo-electric material layer thickens in a transverse direction and contracts in a lateral direction. The membrane however is not contracting and as a result the piezo-electric material near the bottom electrode and the membrane experiences more resistance to contraction than the piezo-electric material near the top electrode. As a result, the piezo-electric actuator bends.
In known bimorph piezo-eletric actuators, the actuator bends towards the membrane. As the membrane commonly forms the flexible wall of the pressure chamber, the volume of the pressure chamber becomes smaller when the bimorph actuator is actuated. On the other hand, for expelling a droplet, the volume is commonly first increased and then the volume is suddenly decreased. In order to enable such operation, a bias-voltage is applied over the bottom electrode and the top electrode when the print head is in a stand-by state. Then, when a droplet needs to be expelled, the bias voltage is lowered (thereby increasing the pressure chamber volume) and then an actuation voltage is applied for decreasing the pressure chamber volume. The actuation voltage may have a same voltage level as the bias voltage or it may have another voltage level. In the latter case, after expelling the droplet, the voltage over the bottom electrode and the top electrode is again brought to the level of the bias voltage or, if another droplet needs to be expelled, it may be lowered again.
A disadvantage of the known bimorph actuator is the need for the bias voltage. Applying a bias voltage results in a deformed actuator, including corresponding stresses in the different layers of the actuator. Ultimately, these stresses shorten the lifetime of the actuator. Further, the application of the bias voltage requires dedicated driver electronics, which dissipate energy and thus generate heat while providing for the bias voltage.